The internal combustion engines of some vehicles are designed to operate with a variety of fuels. In the past, more common applications of multi-fueled vehicle technology were in military settings, where normally-used diesel or gas fuel might not be available during combat operations. However, the growing movement to establish alternatives to automobiles running solely on gasoline has increased the availability of automobiles that use multi-fuel engines, such as bi-fuel or flexible-fuel vehicles.
As a particular example, compression ignition engines are often designed for use with different types of fuels. However, this fuel versatility can be problematic because optimum engine operation tends to call for specificity with regard to fuel viscosity. For example, an engine whose fuel pumps and injectors are designed to use a certain level of viscosity will encounter difficulty operating with fuels having different viscosities.